Mercaptan recovery



Nov. 18, 1941-. T. F. M'uccoRMlcK llrrAL u 2,263,043

MERCAPTAN RECOVERY S sheets-sheet 1 Nov; 18, 1941. T. MccoRMl'cK ALMERCAPTAN RECOVERY -GriginalFiled Dec. .27, 1955 3 Sheets-Sheet 2 0% wwwwww Nov. 18, 1941.

T. FJ MCCORMICK ETAL MERGAPTAN RECOVERY '3 She-ets-Sheet' 5 OriginalFiled Dec. 27. 1935 TTOIENEY Patented Nov. 18, 1941 UNITED STATES'or-rios MERCAPTAN nEcovnaY Thomas F. McCormick, Oakland, and ArthurLazar, Berkeley, Calif., assignors to Tide Water Associated Oil Company,San Francisco, Calif., a corporation of Delaware Original applicationDecember 27, 1935, Serial No.

Divided and this application November 14, 1938, Serial No. 240,246 l 5Claims. (Cl. Zini-609) This invention relates'` to improvements indesulphurizing petroleum or its distillates and has for a particularobject the removal of mercaptans from cracked distillate, especiallycracked gasoline.

Another object is the recovery of mercaptans` butane substantially freefrom HzS, from cracked products. l 'i Further objects will becomeapparent as the invention hereinafter is more fully disclosed.y Afeature ofthe invention comprises the removal of certain compoundscontained in cracked gasoline `which have a deleterious effect onr theselective removal of mercaptans` whereby the mercaptans are left in acondition easily susceptible of removal by themselves.l

Thiswill be better understood by stating that when mixtures oforganiccompounds, such as straight run gasoline, cracked gasolineandother hydrocarbon. mixtures;` are treated with an alkaline reagent, suchascaustic soda'solution, sodium carbonate solution and the'like, areaction with certain acidic compounds dissolved in theoil takes place.l

Such acidic compounds may include hydrogen sulfide and mercaptans.`inasmuch as HzS isaV relatively strong acid and is relativelyeasilyremoved from the oil, such easy removal con-` stitutes a` base towork from leading to the selective removal of less lstrong acidicmercaptans.

The primary step includes the substantially complete` removal of HZS,preferably by the use of steps involving stabilization and fractionationof the distillate'under treatment, because it has been determined thatthe major portion of the mercaptans present in gasolines, or crackedgasolines, is contained in a fractional part thereof, such as the first30% to 40% of the distillation rangeand, for instance, between 170 and250 F. end boiling point and therefore a combined treatment is veryeffective for H25 removal and stabilization for eliminating the use ofchemicals.

`In accordance with this invention, therefore, a

gasoline or total `cracked gasoline is split into` two orl morefractions, according to'volatlity, to

yield a light fraction comprisingabout 20% to 50% of the crackedgasoline and a heavy fraction comprising from about 80% to 50%. This maybe termed a split treat, and the said light fraction may be treatedforthe removal and rel covery of mercaptans so as to requirev a relativelylight sweetening treatment thereafter. l

`The 'invention comprises a method of treatment with respect to HzSremoval without the use of chemicals which will be described.

While the treatment of cracked gasolines is also particularly described,it must be understood that other forms of petroleum distillates, coaltar v distillates, shale oil distillates, or other hydrothe scope of theinvention.

In the appended drawings, Figure 1` is a decarbons or mixtures of thesame, are'included in scriptive flow sheet showing the treatment of boththe said light and heavy fractions.

'Figure 2 is a diagrammatic flofwsheet specifi`` cally illustrating thetreatment of the said light fraction for removal and recovery ofmercaptans,

` heavy fractions.

and regeneration of the treating agent.

Figure 3 is a diagrammatic flow"v sheet illustrating a preferred methodof removing I-IzS from a distillate coupled with efficient fractionationthereof to provide the desired split stream for the light and heavyfractions which arethereby also stabilized.

m Figure l is self explanatory and discloses the variations in`treatments given the light and In Figure 2` the treatment of a lightfraction is seen to comprise ageneral method of .treating and includesthe `passage of total cracked distillate from any Well known crackingand fractionating system 5 through a stabilizer column' B occurs, suchas HzS gas, methane, ethanapropane, etc., said vapors and gases beingeliminated throughvalve l.

Y The liquid distillate, comprising a total cracked" gasoline, thenpasses through pipe 8 into a still, or fractionater 9, wherein a desiredcut is made preferably to yield a light fraction of about 20%" Hence, byfractionating out the low boiling n constituents of a gasoline, orcrackedgasoline,

up to about 40% thereof and treating this fraction separately withcaustic solutions, a maximum removal of mercaptans is assured togetherwith maximum desulphurization.

And, not only are the above factors important, but the removal ofmercaptans by themselves relieves the load ordinarily thrown on thedoctor treatment for sweetening wherein the mercaptans are altered todisulphides, and also the caustic solution is capable of regenerationfor reuse for several cycles.

In addition to the above described treatment for selective removal ofHzS whereby a large percentage ofthe mercaptans remain dissolved in thelight fraction, it is found that temperature has a marked effect on theequilibrium condition and that as the temperature of the mecaptancontaining light fraction mixed with caustic soda solution approachesthe freezing point of the said solution the solubility of the mercaptansin caustic increases in proportion.

In consequence, the treated light fraction issuing from tower 20 ispassed through line 29 to a third similarly constructed contact tower30, being cooled by cooler 3| to a point preferably just above thefreezing point of a caustic solution introduced into tower 30v by pump32 from the bottom of a fourth similarly constructed contact tower 38through lines 33, 34, and spray 35,

The caustic solution then passes in true countercurrent flow to thelight fraction through tower 30, the spent caustic containing mercaptansbeing withdrawn to tank 36, while the light fraction relieved of part ofthe mercaptans iiows through line 3l to a fourth similarly constructedcontact tower 33, being cooled, as before, on its way by cooler 39 to apoint preferably just above the freezing point of fresh strong causticsolution suppliedk through 'line 40, cooler 4l, and spray 42.

In tower 38, contact of the once treated cooled light fraction iscompleted with fresh caustic to finally remove all mercaptans in thecaustic in the form of sodium mercaptides as far as is possible underthe optimum conditions of temperature and strength of soda solution.

In order to assure the most eflicient contact, pump 32 provides a rateof circulation from tower 38 back to tower 30 at a rate up to about 50%,or higher, of the volume of the light fraction under treatment, which isnally withdrawn through line 43 to undergo the usual doctor treatmentfor sweetening and is Athen a finished light fraction ready for blendingto yield a finished motor fuel. Some of the advantages of thus treatingthe light fraction may be summarized as follows:

(1) The unsaturates present in the light fraction which are highlydesired for their antiknock value in finished gasolines are not attackedand lost as in sulphuric acid treatment.

(2) A re-run distillation with its accompanying losses may, in manycases, be dispensed with. due to the elimination of polymerization whichtakes place in ordinary acid treatment.

(3) The loss in octane number is not only minimized when compared withacid treatment followed by distillation but the octane value may beactually increased.

(4) The desulphurization by mercaptan removal is a maximum with theselective treatment of a light fraction.

(5) The maximum value may be obtained from CII the caustic soda solutionby regeneration and re-use.

The temperature to which the reacting mixture is reduced in towers 30and 38 may be just above the freezing point of the caustic solution usedup to F., and the amount of caustic'soda solution used ranges from about5% to 30% by volume of the light fraction treated and is of a strengthfrom about 3 to 20 B. Preferably, the caustic strength used is between 5and 12 B., and about 10% by volume and the treating temperature intowers 30 and 38 and 30 to 40 F.

Caustic soda solution may be replaced by caustic potash, as is wellknown.

The efliciency of the process as a whole, while not dependent thereon,is materially increased by the regeneration and reuse of the spentcaustic solution from tower 30, and the regeneration may be accomplishedby either batch or continuous operation. While batch regeneration iseffective, continuous regeneration is to be preferred in order toconform with the continuous operation of the preceding steps set forth.

In the batch regeneration, by reference to Figure 2, it will be seenthat the spent caustic containing sodium mercaptides is drawn from tank36 through line 44 to tank 45 containing a closed steam coil 46 and anopen line 41 for the introduction of air, steam, or an inert gas forstripping.

A solution level X-X gives head room for the removal of the low boilingmercaptans in vapor form through pipe 4B, the same being derived by thesplitting up of the sodium mercaptides under the influence of heatapplied through coil 46 to bring the solution to or near, the boilingpoint.

Alternative methods of applying open steam air, etc., may be used. Ifsteam alone be passed through the mixture, the mercaptan sulphur may bereduced from say 50 grams per liter to as low as 2 grams per liter. Ifthe velocity of the steam through the mixture is great enough to causefoaming, additional steam may be passed through spray line 49 to breakthe foam.

Alternatively and preferably, continuous regeneration is effected bypassing` the spent caustic solution from tank 35 through line 44 andpump 50 to the top of a contact tower 5| containing a plurality ofbubble plates 52, the caustic solution flowing down countercurrent 'toythe heating and stripping steam or gas supplied through line 53.

In such a contact tower 5l, when insulated and being 4 in diameter and32 high, using 1.8 pounds of steam for 5 pounds of caustic solutioncharged, it is found that with seven plates the mercaptan sulphurcontent will be reduced from 40 grams per liter to 6 grams per liter.Using a higher ratio of steam to caustic the mercaptan content may bestill further reduced, or the number of plates may be increased toprovide an additional reduction in mercaptan sulphur.

The mercaptans are taken as overhead through line 54 t0 an insulatedslop pot 55, wherein a separation of vapors from any caustic in the formof foam takes place, and the mercaptan vapors then pass through line 51to a contact tower 58 containing suitable Contact material 59 in whichthe vapors are scrubbed with water applied through pipe 30 to condensethe mercaptans at a predetermined temperature and pressure.

Vapors are tapped off through line 6| and are composed largely of themore volatile mercaptans as methyl and ethyl mercaptan. 'I'hese'mercaptans may be recovered by suitable absorption agents or may beliquefied by means of refrigeration or compression in the well knownmanner. Mercaptans condensedby the water pass to a settling tank 62wherein the water separates by gravity to be withdrawn through pipe 63controlled b-y an automatic liquid level regulator 64. The desiredliquid mercaptans are drawn off through pipe 65.` r

Any caustic solution passed over fromtower and trapped out in the sloppot 55 may be returned by pump 56 for restripping in tower 5I.

The 'regenerated caustic solution flowscontinuously from tower 5I1intotank liiwhence it maybe withdrawn through line 61 to be used as chargingcaustic through line 40. In similar the batch treatment (when used) maybe passed through line 68 to line 40.

The life of lthe regenerated caustic solution is not necessarilyindefinite as there may be a gradual accumulation of impurities in thesame which are not responsive to regenerative separation, but it may beused effectively in towers 38and 30 forat least three cycles which gives`great economy in caustic consumption.

` Referring to Figure 3, a preferred method of i fractions is shown.

In these steps, which are preferably used in i fashion, the regeneratedcaustic solution from i place of the stabilizer column 6 and still 9 ofFigure 2, the iiow sheet and data represent actual renery operations`during the production of nished cracked gasoline from a raw crackeddistillate having the following characteristics:

Gravity 581.2

Sulphur per cent .'15

Octane number (C. F. R. motor method) 70 Initial B. point 90 Per cent:

End point 399 Recovery v 95.0!

Residue 1.0

Loss 4.0

The raw cracked distillate frointhe cracking stills is forced by pump 10through line |l`into a ilash tower l2, absorbing `heat during itspassage from partial condenser '|3,` heat exchangers 1.4 and heater 15so that the distillate is discharged into flash tower 12 at about 362F., or

at 362 F. and under a pressure of 265 pounds per square inch, gage, thedesired light fraction per liter inthe bottoms which are withdrawnthrough line `'I8 4and passed into a primary tower '|9. Said bottoms areinjected into primary tower 19 at a temperatureof about 310 F. forrectification by removal ofthe remaining I-I2S and light ends of theboiling range of the desired light fraction. To this end, primary towerI9 may be a 30 `plate tower, fitted with a reboiler 80 wherein thebottoms are heated to about 375` F., operated under a pressure of about40 pounds per square inch, gage, and supplied with suitable refluxliquid through line 8| which may conveniently `be supplied from anaccumulator 82 wherein is collected part of the overhead from primarytower 1.9 which passes thereto by way of line 83 and partialcondensers'|3 and 84. lIn primary tower 10, operating under the above describedconditions, the I-IaS will be reduced from 0.09 gram per liter of theentering stock to a trace in the rectified heavy fraction passing outthrough une a5 while `the overhead (comprising constituents of thedesired light fraction) passes from accumulator '82 through line 86 andcondenser 81 to be collected in accumulator 88 in from flash tower 12through line 93.

Secondary tower 92 is provided for the purpose of stabilizing thedesired light fraction by the elimination of undesired wild gases such,as

methane, ethane, etc., during which operation all -I-IZS is separated.To this end, tower 92 "is i u provided with a reboiler 94adapted to heatthe bottoms to a temperature of about 300 F. there by giving atemperature gradient through the column and yield an overhead having atemperature of about 120 F. at the outlet pipe 95'while operating undera pressure of about 245 pounds per squareinch, gage.

The overhead from secondary tower 92 is taken through line 95 andcondenser 90 to provide condensate in accumulator 91 which is forced bypump 98 through line 99 as reflux in the upper part of the tower. Excesscondensate may be taken through line I 00 to be used as blendingpropane, or for other purposes, while the incondensible wild gases andI-IZS through line |0|.

The light fraction is passed at ,a temperature of about 300 F. throughline |03 into a 24 plate butane tower |04 wherein a cut of commercialare withdrawn butane is taken as overhead, the bottomscom t5` prisingthe light fraction within the gasoline boiling range being withdrawnthrough lines |05 and |02 for any desired treatment.

A` cut comprising substantially butanes `and hydrocarbons of'similarboiling range is desired with, for example, an absolute vapor pressureofabout 24 to 75 pounds persquare inch at '70 F., tower |04 may beprovided with a reboiler |06 giving a bottom temperature of about`220`and a temperature at outlet line I 0`|of about'120" "F.whileoperatinglunder pressure of about v Mol. per cent Propane 0. 2 Isobutane22. 3 Isobutylene c 15.2 Normal butylene l 23. 8 Butadiene 2. 0 Normalbutane 33.1 Methyl mercaptan 0.3 Isopentane and heavier 3.1

rl'he overhead passing through line |01', reflux oondensers |08 andheadcondenser |08', will ow as a liquid through line Ill), part beingused as reflux through line IH and the remainder flowing through line H2and coolers H3 to a treating system for the recovery of methyl mercaptanin substantially pure form. The incondensible gases are withdrawnthrough line H4.

It is foundthat by thus running a raw cracked distillate, as described,in the butane cut mercaptans are present up to about 0.35%' by weightmercaptan sulphur and that the mercaptan sulphur .is derived from thespecic mercaptan, methyl mercaptan CHsSH, which is removed by causticsoda solutions according to the following equation:NaOH-l-.CI-IaSHzCHaSNa-i-H2O.

Inconsequence, the butane cut from tower IM through line H2 is treatedwith causticsoda solution accord-ing to the steps loutlined with respectto the operation of towers 3,0 and 38 in Figure 2. In this case, towers3,0 and 38 will be supplied with cooled caustic` soda solution of about5 to 12 Baume strength and the cooled butane will flow countercurrent tothe caustic with eflicient contacting due to the structure of thetowers, the reactionbeing conducted atl about 30 to 40 F.

Thel spent caustic may then pass, as described in connection with Figure2, from tower 30 to the described continuous 4regeneration plant whenpure methyl mercaptan may be recovered from line 65. l

More simply, the methyl mercaptan may be removed from the spent causticby' boiling the latter and condensing the distilled mercaptan, or bypassing steam, air, or other inert stripping gas through the hot causticsolution-and condensing the distilled mercaptan.-

The preferred system, above outlined, for obtaining desired light andheavy fractions for separate treatments in the removal of impurities hasmany advantages.

It provides for the complete removal of HzS without recourse to thechemical treatment usually necessary. l

It gives improved fractionation for the desired split wherein gaps ofover 20 F. between the A. S. T. M. Engler end point of the lightfraction and the Engler initial boiling point of the heavy frac-tion canbe obtained, which is important when a concentration of mercaptans hasbeen determined in agiven percentage of the raw distillate and thuslimits the low boiling sulphur compounds to the light fraction.

It gives by improved fractionation a light fraction of a desired boilingrange which is largely uncontaminated with higher 'boiling and morerefractory types of sulphur compounds or with compounds which may impaircolor stability.

It minimizes the use and cost of chemicals while yielding relativelypure valuable by-products. i

It gives a commercial butane free from HzS.

It yields a speciic mercaptan in relatively pure form.

It must be understood that the preferred method of Figure 3 has beenapplied to the treatment of a cracked gasoline specifically for thepurpose of illustration and not of limitation, inasmuch as variations intemperature, pressure, tower construction and the like are permissibledepending on the required split in the boiling ranges of the raw crackeddistillate, the character of the cracked gasoline and many'othervariables well known in the art.

For instance, in the removal of H255 alone, the operation of flash towerl2 is not as important as its chief function which is the stripping ofthe lightest hydrocarbons from the raw distillate so that the load onthe primary tower 'I9 is reduced and thus permits condensation ofoverhead vapors therefrom at ordinary cooling water temperatures andcomparatively low pressures. This permits the use of a less costlyprimary tower 'i9 and cuts down the use and cost of steam used inreboiler 8G and the cost of condensing the overhead vapors with coolingmediums other than water.

Again, while correct control and adjustment of the operations inprima-ry tower i9 and secondary tower 92 are important for the removalof Hes the iigures given for a particular cracked distillate may bevaried to suit conditions. For instance, if it is desired to operatethese towers at higher pressures, the top and bottom temperatures mustbe raised or, vice versa, if the pressure is reduced, the temperaturesshould be reduced.

A great many of such variables follow physical laws, the operation ofwhich is well known to those skilled in the art, and the practice of theinvention requires no other instructions to permit its use beyond thosegiven. Ordinary laboratory controls to determine characteristics of aparticular distillate will govern correct refinery control of theprocess.

The definition of the term total cracked distillate used hereinindicates a distillate derived from cracking various stocks and isusually a distillate cut to an end point by A. S. T. M. distillationsimilar to normal motor fuel marketed. However, the total crackedgasoline may have an end point from 250 F. to 500 F., or higher,depending entirely on the particular fractionating process and/orequipment used As a rule, the end point on total cracked gasoline is cutwith modern methods and equipment to somewhere between 350 F. and 437F., but variations from this range are permissible within the scope ofthe invention and may include distillates from other sources such, forinstance, as

those made by polymerization of hydrocarbon gases as Well as sulphurbearing distillates from other sources.

In order to obtain products in the most purii fled form, hydrogensulphide should lbe rst reapplication Serial No. 56,398, led December27,

1935, now Patent No. 2,183,968, Dec. 19, 1939.

We claim:

1. The method of recovery of pure methyl mercaptan which comprises:passing a hydrocarbon distillate containing H25, methyl mercaptan andother acidic compounds into a first fractionating zone wherein saiddistillate is fractionated into a light fraction containingsubstantially all said l-lzS and said methyl mercaptan and a heavyfraction comprising from about 80% to 50% of the distillate, passingsaid light frac-- 2. The method ci recovery of pure methyl mercaptanwhich comprises: passing a raw cracked gasoline containing HzS, methylmercaptan and other acidic bodies into a rst fractionating zone whereinis removed as overhead up to of the lighter constitutents of thegasoline, passing the bottoms from the rst fractionating zone into asecond fractionating zone operated under such conditions of temperatureand pressure as to remove as overhead a fraction containingsubstantially all the remaining I-IzS and methyl mercaptan, combiningthe overheads from said first zone and said second zone, passing saidcombined overheads comprising up to 50% of said gasoline into a thirdfractionating zone operated under such conditions of temperature andpressure as to remove as overhead all the HzS together with hydrocarbonshaving a boiling point below isobutane, passing the bottoms from saidthird zone to a fourth fractionating zone operated under such conditionsof temperature and pressure as to remove as overhead a fraction of theapproximate boiling range of butane containing methyl mercaptan and freefrom other acidic bodies, treating said overhead with an aqueous alkalisolution, and recovering methyl mercaptan from the reacted alkali.

3. The method of recovery of pure methyl mercaptan which comprises:passing a raw cracked gasoline containing HzS, methyl mercaptan andother acidic bodies into a first fractionating zone wherein is removedas overhead up to 25% of the lighter constituents ofthe gasoline,passing the bottoms from the rst fractionating zone into a secondfractionating zone operated under such conditions of temperature andpressure as to remove as overhead a fraction containing substantiallyall the remaining HzS and methyl mercaptan, supplying reflux to said rstzone and to said second Zone from condensed overhead from said secondzone, combining the overheads from said rst zone and said second zone,passing said combined overheads comprising up to 50% of said gasolineinto a third fractionating zone operated under such conditions oftemperature and pressure as to remove as overhead all the HzS tqgetherwith hydrocarbons having a boiling point below isobutane, supplyingreflux to said third fractionating zone from partially condensedoverhead from said third zone, passing the bottoms from said third zoneto a fourth fractionating zone operated under such conditions oftemperature and pressure as to remove as overhead a fraction of theapproximate boiling range of butane containing methyl mercaptan and freefrom other acidic bodies, treating said overhead with an aqueous alkalisolution, and recovering methyl mercaptan from the reacted alkali.

4. In a process wherein a butane fraction obtained from a crackedhydrocarbon distillate is treated for the recovery of methyl mercaptan,the method of preparing such a fraction free of I-IzS prior to suchmercaptan recovery which comprises: passing a raw cracked distillatecontaining HzS, methyl mercaptan, and other acidic compounds into a rstfractionating zone wherein said distillate is fractionated into a lightfraction containing substantially all said HzS and said methyl mercaptanand a heavy fraction comprising from about to 50% of the distillate,passing said light fraction into a second fractionating zone wherein isseparated all H2S and the major portion of wild gases, and passing thebottoms from said second fractionating zone into a third fractionatingZone wherein is obtained as overhead a fraction containing the majorportion of the methyl mercaptan but free from other acidic bodies andhaving 'a boiling range approximately that of butane.

5. In the stabilization of cracked distillates suitable in boiling rangefor motor fuel and containing I-IzS and mercaptans, the process whichcomprises: splitting such a cracked distillate by fractionation toremove from 20% to 50% of the distillate' as a fraction containingsubstantially all the HzS in the distillate together with methylmercaptan, fractionating said fraction to remove the I-I2S and leave themercaptan in the bottoms, then fractionating said bottoms to remove themercaptan as overhead.

THOMAS F. MCCORlWICK. ARTHUR LAZAR.

